A crude table of contents for the Perl manpage set is found in perltoc.

How can I use Perl interactively?

The typical approach uses the Perl debugger, described in the
perldebug(1) manpage, on an ``empty'' program, like this:

perl -de 42

Now just type in any legal Perl code, and it will be immediately
evaluated. You can also examine the symbol table, get stack
backtraces, check variable values, set breakpoints, and other
operations typically found in symbolic debuggers.

You can also use Devel::REPL which is an interactive shell for Perl,
commonly known as a REPL - Read, Evaluate, Print, Loop. It provides
various handy features.

How do I find which modules are installed on my system?

From the command line, you can use the "cpan" command's "-l" switch:

$ cpan -l

You can also use "cpan"'s "-a" switch to create an autobundle file
that "CPAN.pm" understands and can use to re-install every module:

$ cpan -a

Inside a Perl program, you can use the ExtUtils::Installed module to
show all installed distributions, although it can take awhile to do
its magic. The standard library which comes with Perl just shows up
as ``Perl'' (although you can get those with Module::CoreList).

If you simply need to check quickly to see if a module is
available, you can check for its documentation. If you can
read the documentation the module is most likely installed.
If you cannot read the documentation, the module might not
have any (in rare cases):

$ perldoc Module::Name

You can also try to include the module in a one-liner to see if
perl finds it:

$ perl -MModule::Name -e1

(If you don't receive a ``Can't locate ... in @INC'' error message, then Perl
found the module name you asked for.)

How do I debug my Perl programs?

(contributed by brian d foy)

Before you do anything else, you can help yourself by ensuring that
you let Perl tell you about problem areas in your code. By turning
on warnings and strictures, you can head off many problems before
they get too big. You can find out more about these in strict
and warnings.

#!/usr/bin/perl
use strict;
use warnings;

Beyond that, the simplest debugger is the "print" function. Use it
to look at values as you run your program:

Perl comes with an interactive debugger, which you can start with the
"-d" switch. It's fully explained in perldebug.

If you'd like a graphical user interface and you have Tk, you can use
"ptkdb". It's on CPAN and available for free.

If you need something much more sophisticated and controllable, Leon
Brocard's Devel::ebug (which you can call with the "-D" switch as "-Debug")
gives you the programmatic hooks into everything you need to write your
own (without too much pain and suffering).

You can also use a commercial debugger such as Affrus (Mac OS X), Komodo
from Activestate (Windows and Mac OS X), or EPIC (most platforms).

How do I profile my Perl programs?

(contributed by brian d foy, updated Fri Jul 25 12:22:26 PDT 2008)

The "Devel" namespace has several modules which you can use to
profile your Perl programs.

The Devel::NYTProf (New York Times Profiler) does both statement
and subroutine profiling. It's available from CPAN and you also invoke
it with the "-d" switch:

perl -d:NYTProf some_perl.pl

It creates a database of the profile information that you can turn into
reports. The "nytprofhtml" command turns the data into an HTML report
similar to the Devel::Cover report:

nytprofhtml

You might also be interested in using the Benchmark to
measure and compare code snippets.

You can read more about profiling in Programming Perl, chapter 20,
or Mastering Perl, chapter 5.

How do I cross-reference my Perl programs?

The B::Xref module can be used to generate cross-reference reports
for Perl programs.

perl -MO=Xref[,OPTIONS] scriptname.plx

Is there a pretty-printer (formatter) for Perl?

Perl::Tidy comes with a perl script perltidy which indents and
reformats Perl scripts to make them easier to read by trying to follow
the rules of the perlstyle. If you write Perl, or spend much time reading
Perl, you will probably find it useful.

Of course, if you simply follow the guidelines in perlstyle,
you shouldn't need to reformat. The habit of formatting your code
as you write it will help prevent bugs. Your editor can and should
help you with this. The perl-mode or newer cperl-mode for emacs
can provide remarkable amounts of help with most (but not all)
code, and even less programmable editors can provide significant
assistance. Tom Christiansen and many other VI users swear by
the following settings in vi and its clones:

set ai sw=4
map! ^O {^M}^[O^T

Put that in your .exrc file (replacing the caret characters
with control characters) and away you go. In insert mode, ^T is
for indenting, ^D is for undenting, and ^O is for blockdenting---as
it were. A more complete example, with comments, can be found at
<http://www.cpan.org/authors/id/TOMC/scripts/toms.exrc.gz>

Is there an IDE or Windows Perl Editor?

Perl programs are just plain text, so any editor will do.

If you're on Unix, you already have an IDE---Unix itself. The Unix
philosophy is the philosophy of several small tools that each do one
thing and do it well. It's like a carpenter's toolbox.

If you want an IDE, check the following (in alphabetical order, not
order of preference):

GUI editor written in Perl using wxWidgets and Scintilla with lots of smaller features.
Aims for a UI based on Perl principles like TIMTOWTDI and ``easy things should be easy,
hard things should be possible''.

Zeus for Windows is another Win32 multi-language editor/IDE
that comes with support for Perl.

For editors: if you're on Unix you probably have vi or a vi clone
already, and possibly an emacs too, so you may not need to download
anything. In any emacs the cperl-mode (M-x cperl-mode) gives you
perhaps the best available Perl editing mode in any editor.

If you are using Windows, you can use any editor that lets you work
with plain text, such as NotePad or WordPad. Word processors, such as
Microsoft Word or WordPerfect, typically do not work since they insert
all sorts of behind-the-scenes information, although some allow you to
save files as ``Text Only''. You can also download text editors designed
specifically for programming, such as Textpad (
<http://www.textpad.com/> ) and UltraEdit ( <http://www.ultraedit.com/> ),
among others.

There is also a toyedit Text widget based editor written in Perl
that is distributed with the Tk module on CPAN. The ptkdb
( <http://ptkdb.sourceforge.net/> ) is a Perl/Tk-based debugger that
acts as a development environment of sorts. Perl Composer
( <http://perlcomposer.sourceforge.net/> ) is an IDE for Perl/Tk
GUI creation.

In addition to an editor/IDE you might be interested in a more
powerful shell environment for Win32. Your options include

Where can I get perl-mode or cperl-mode for emacs?

Since Emacs version 19 patchlevel 22 or so, there have been both a
perl-mode.el and support for the Perl debugger built in. These should
come with the standard Emacs 19 distribution.

Note that the perl-mode of emacs will have fits with "main'foo"
(single quote), and mess up the indentation and highlighting. You
are probably using "main::foo" in new Perl code anyway, so this
shouldn't be an issue.

How can I use curses with Perl?

The Curses module from CPAN provides a dynamically loadable object
module interface to a curses library. A small demo can be found at the
directory <http://www.cpan.org/authors/Tom_Christiansen/scripts/rep.gz> ;
this program repeats a command and updates the screen as needed, rendering
rep ps axu similar to top.

How can I write a GUI (X, Tk, Gtk, etc.) in Perl?

(contributed by Ben Morrow)

There are a number of modules which let you write GUIs in Perl. Most
GUI toolkits have a perl interface: an incomplete list follows.

Tk

This works under Unix and Windows, and the current version doesn't
look half as bad under Windows as it used to. Some of the gui elements
still don't 'feel' quite right, though. The interface is very natural
and 'perlish', making it easy to use in small scripts that just need a
simple gui. It hasn't been updated in a while.

Wx

This is a Perl binding for the cross-platform wxWidgets toolkit
( <http://www.wxwidgets.org> ). It works under Unix, Win32 and Mac OS X,
using native widgets (Gtk under Unix). The interface follows the C++
interface closely, but the documentation is a little sparse for someone
who doesn't know the library, mostly just referring you to the C++
documentation.

Gtk and Gtk2

These are Perl bindings for the Gtk toolkit ( <http://www.gtk.org> ). The
interface changed significantly between versions 1 and 2 so they have
separate Perl modules. It runs under Unix, Win32 and Mac OS X (currently
it requires an X server on Mac OS, but a 'native' port is underway), and
the widgets look the same on every platform: i.e., they don't match the
native widgets. As with Wx, the Perl bindings follow the C API closely,
and the documentation requires you to read the C documentation to
understand it.

Win32::GUI

This provides access to most of the Win32 GUI widgets from Perl.
Obviously, it only runs under Win32, and uses native widgets. The Perl
interface doesn't really follow the C interface: it's been made more
Perlish, and the documentation is pretty good. More advanced stuff may
require familiarity with the C Win32 APIs, or reference to MSDN.

CamelBones

CamelBones ( <http://camelbones.sourceforge.net> ) is a Perl interface to
Mac OS X's Cocoa GUI toolkit, and as such can be used to produce native
GUIs on Mac OS X. It's not on CPAN, as it requires frameworks that
CPAN.pm doesn't know how to install, but installation is via the
standard OSX package installer. The Perl API is, again, very close to
the ObjC API it's wrapping, and the documentation just tells you how to
translate from one to the other.

Qt

There is a Perl interface to TrollTech's Qt toolkit, but it does not
appear to be maintained.

Athena

Sx is an interface to the Athena widget set which comes with X, but
again it appears not to be much used nowadays.

How can I make my Perl program run faster?

The best way to do this is to come up with a better algorithm. This
can often make a dramatic difference. Jon Bentley's book
Programming Pearls (that's not a misspelling!) has some good tips
on optimization, too. Advice on benchmarking boils down to: benchmark
and profile to make sure you're optimizing the right part, look for
better algorithms instead of microtuning your code, and when all else
fails consider just buying faster hardware. You will probably want to
read the answer to the earlier question ``How do I profile my Perl
programs?'' if you haven't done so already.

A different approach is to autoload seldom-used Perl code. See the
AutoSplit and AutoLoader modules in the standard distribution for
that. Or you could locate the bottleneck and think about writing just
that part in C, the way we used to take bottlenecks in C code and
write them in assembler. Similar to rewriting in C, modules that have
critical sections can be written in C (for instance, the PDL module
from CPAN).

If you're currently linking your perl executable to a shared
libc.so, you can often gain a 10-25% performance benefit by
rebuilding it to link with a static libc.a instead. This will make a
bigger perl executable, but your Perl programs (and programmers) may
thank you for it. See the INSTALL file in the source distribution
for more information.

The undump program was an ancient attempt to speed up Perl program by
storing the already-compiled form to disk. This is no longer a viable
option, as it only worked on a few architectures, and wasn't a good
solution anyway.

How can I make my Perl program take less memory?

When it comes to time-space tradeoffs, Perl nearly always prefers to
throw memory at a problem. Scalars in Perl use more memory than
strings in C, arrays take more than that, and hashes use even more. While
there's still a lot to be done, recent releases have been addressing
these issues. For example, as of 5.004, duplicate hash keys are
shared amongst all hashes using them, so require no reallocation.

In some cases, using substr() or vec() to simulate arrays can be
highly beneficial. For example, an array of a thousand booleans will
take at least 20,000 bytes of space, but it can be turned into one
125-byte bit vector---a considerable memory savings. The standard
Tie::SubstrHash module can also help for certain types of data
structure. If you're working with specialist data structures
(matrices, for instance) modules that implement these in C may use
less memory than equivalent Perl modules.

Another thing to try is learning whether your Perl was compiled with
the system malloc or with Perl's builtin malloc. Whichever one it
is, try using the other one and see whether this makes a difference.
Information about malloc is in the INSTALL file in the source
distribution. You can find out whether you are using perl's malloc by
typing "perl -V:usemymalloc".

Of course, the best way to save memory is to not do anything to waste
it in the first place. Good programming practices can go a long way
toward this:

Don't slurp!

Don't read an entire file into memory if you can process it line
by line. Or more concretely, use a loop like this:

#
# Good Idea
#
while (my $line = <$file_handle>) {
# ...
}

instead of this:

#
# Bad Idea
#
my @data = <$file_handle>;
foreach (@data) {
# ...
}

When the files you're processing are small, it doesn't much matter which
way you do it, but it makes a huge difference when they start getting
larger.

Use map and grep selectively

Remember that both map and grep expect a LIST argument, so doing this:

@wanted = grep {/pattern/} <$file_handle>;

will cause the entire file to be slurped. For large files, it's better
to loop:

while (<$file_handle>) {
push(@wanted, $_) if /pattern/;
}

Avoid unnecessary quotes and stringification

Don't quote large strings unless absolutely necessary:

my $copy = "$large_string";

makes 2 copies of $large_string (one for $copy and another for the
quotes), whereas

my $copy = $large_string;

only makes one copy.

Ditto for stringifying large arrays:

{
local $, = "\n";
print @big_array;
}

is much more memory-efficient than either

print join "\n", @big_array;

or

{
local $" = "\n";
print "@big_array";
}

Pass by reference

Pass arrays and hashes by reference, not by value. For one thing, it's
the only way to pass multiple lists or hashes (or both) in a single
call/return. It also avoids creating a copy of all the contents. This
requires some judgement, however, because any changes will be propagated
back to the original data. If you really want to mangle (er, modify) a
copy, you'll have to sacrifice the memory needed to make one.

Tie large variables to disk

For ``big'' data stores (i.e. ones that exceed available memory) consider
using one of the DB modules to store it on disk instead of in RAM. This
will incur a penalty in access time, but that's probably better than
causing your hard disk to thrash due to massive swapping.

Is it safe to return a reference to local or lexical data?

Yes. Perl's garbage collection system takes care of this so
everything works out right.

How can I free an array or hash so my program shrinks?

(contributed by Michael Carman)

You usually can't. Memory allocated to lexicals (i.e. my() variables)
cannot be reclaimed or reused even if they go out of scope. It is
reserved in case the variables come back into scope. Memory allocated
to global variables can be reused (within your program) by using
undef() and/or delete().

On most operating systems, memory allocated to a program can never be
returned to the system. That's why long-running programs sometimes re-
exec themselves. Some operating systems (notably, systems that use
mmap(2) for allocating large chunks of memory) can reclaim memory that
is no longer used, but on such systems, perl must be configured and
compiled to use the OS's malloc, not perl's.

In general, memory allocation and de-allocation isn't something you can
or should be worrying about much in Perl.

See also ``How can I make my Perl program take less memory?''

How can I make my CGI script more efficient?

Beyond the normal measures described to make general Perl programs
faster or smaller, a CGI program has additional issues. It may be run
several times per second. Given that each time it runs it will need
to be re-compiled and will often allocate a megabyte or more of system
memory, this can be a killer. Compiling into C isn't going to help
you because the process start-up overhead is where the bottleneck is.

There are three popular ways to avoid this overhead. One solution
involves running the Apache HTTP server (available from
<http://www.apache.org/> ) with either of the mod_perl or mod_fastcgi
plugin modules.

With mod_perl and the Apache::Registry module (distributed with
mod_perl), httpd will run with an embedded Perl interpreter which
pre-compiles your script and then executes it within the same address
space without forking. The Apache extension also gives Perl access to
the internal server API, so modules written in Perl can do just about
anything a module written in C can. For more on mod_perl, see
<http://perl.apache.org/>

With the FCGI module (from CPAN) and the mod_fastcgi
module (available from <http://www.fastcgi.com/> ) each of your Perl
programs becomes a permanent CGI daemon process.

Finally, Plack is a Perl module and toolkit that contains PSGI middleware,
helpers and adapters to web servers, allowing you to easily deploy scripts which
can continue running, and provides flexibility with regards to which web server
you use. It can allow existing CGI scripts to enjoy this flexibility and
performance with minimal changes, or can be used along with modern Perl web
frameworks to make writing and deploying web services with Perl a breeze.

These solutions can have far-reaching effects on your system and on the way you
write your CGI programs, so investigate them with care.

How can I hide the source for my Perl program?

Delete it. :-) Seriously, there are a number of (mostly
unsatisfactory) solutions with varying levels of ``security''.

First of all, however, you can't take away read permission, because
the source code has to be readable in order to be compiled and
interpreted. (That doesn't mean that a CGI script's source is
readable by people on the web, though---only by people with access to
the filesystem.) So you have to leave the permissions at the socially
friendly 0755 level.

Some people regard this as a security problem. If your program does
insecure things and relies on people not knowing how to exploit those
insecurities, it is not secure. It is often possible for someone to
determine the insecure things and exploit them without viewing the
source. Security through obscurity, the name for hiding your bugs
instead of fixing them, is little security indeed.

You can try using encryption via source filters (Starting from Perl
5.8 the Filter::Simple and Filter::Util::Call modules are included in
the standard distribution), but any decent programmer will be able to
decrypt it. You can try using the byte code compiler and interpreter
described later in perlfaq3, but the curious might still be able to
de-compile it. You can try using the native-code compiler described
later, but crackers might be able to disassemble it. These pose
varying degrees of difficulty to people wanting to get at your code,
but none can definitively conceal it (true of every language, not just
Perl).

It is very easy to recover the source of Perl programs. You simply
feed the program to the perl interpreter and use the modules in
the B:: hierarchy. The B::Deparse module should be able to
defeat most attempts to hide source. Again, this is not
unique to Perl.

If you're concerned about people profiting from your code, then the
bottom line is that nothing but a restrictive license will give you
legal security. License your software and pepper it with threatening
statements like ``This is unpublished proprietary software of XYZ Corp.
Your access to it does not give you permission to use it blah blah
blah.'' We are not lawyers, of course, so you should see a lawyer if
you want to be sure your license's wording will stand up in court.

How can I compile my Perl program into byte code or C?

(contributed by brian d foy)

In general, you can't do this. There are some things that may work
for your situation though. People usually ask this question
because they want to distribute their works without giving away
the source code, and most solutions trade disk space for convenience.
You probably won't see much of a speed increase either, since most
solutions simply bundle a Perl interpreter in the final product
(but see ``How can I make my Perl program run faster?'').

How can I get #!perl to work on [MS-DOS,NT,...]?

For OS/2 just use

extproc perl -S -your_switches

as the first line in "*.cmd" file ("-S" due to a bug in cmd.exe's
``extproc'' handling). For DOS one should first invent a corresponding
batch file and codify it in "ALTERNATE_SHEBANG" (see the
dosish.h file in the source distribution for more information).

The Win95/NT installation, when using the ActiveState port of Perl,
will modify the Registry to associate the ".pl" extension with the
perl interpreter. If you install another port, perhaps even building
your own Win95/NT Perl from the standard sources by using a Windows port
of gcc (e.g., with cygwin or mingw32), then you'll have to modify
the Registry yourself. In addition to associating ".pl" with the
interpreter, NT people can use: "SET PATHEXT=%PATHEXT%;.PL" to let them
run the program "install-linux.pl" merely by typing "install-linux".

Under ``Classic'' MacOS, a perl program will have the appropriate Creator and
Type, so that double-clicking them will invoke the MacPerl application.
Under Mac OS X, clickable apps can be made from any "#!" script using Wil
Sanchez' DropScript utility: <http://www.wsanchez.net/software/> .

IMPORTANT!: Whatever you do, PLEASE don't get frustrated, and just
throw the perl interpreter into your cgi-bin directory, in order to
get your programs working for a web server. This is an EXTREMELY big
security risk. Take the time to figure out how to do it correctly.

Why don't Perl one-liners work on my DOS/Mac/VMS system?

The problem is usually that the command interpreters on those systems
have rather different ideas about quoting than the Unix shells under
which the one-liners were created. On some systems, you may have to
change single-quotes to double ones, which you must NOT do on Unix
or Plan9 systems. You might also have to change a single % to a %%.

The problem is that none of these examples are reliable: they depend on the
command interpreter. Under Unix, the first two often work. Under DOS,
it's entirely possible that neither works. If 4DOS was the command shell,
you'd probably have better luck like this:

perl -e "print <Ctrl-x>"Hello world\n<Ctrl-x>""

Under the Mac, it depends which environment you are using. The MacPerl
shell, or MPW, is much like Unix shells in its support for several
quoting variants, except that it makes free use of the Mac's non-ASCII
characters as control characters.

Using qq(), q(), and qx(), instead of ``double quotes'', 'single
quotes', and `backticks`, may make one-liners easier to write.

There is no general solution to all of this. It is a mess.

[Some of this answer was contributed by Kenneth Albanowski.]

Where can I learn about CGI or Web programming in Perl?

For modules, get the CGI or LWP modules from CPAN. For textbooks,
see the two especially dedicated to web stuff in the question on
books. For problems and questions related to the web, like ``Why
do I get 500 Errors'' or ``Why doesn't it run from the browser right
when it runs fine on the command line'', see the troubleshooting
guides and references in perlfaq9 or in the CGI MetaFAQ:

Looking in to Plack and modern Perl web frameworks is highly recommended,
though; web programming in Perl has evolved a long way from the old days of
simple CGI scripts.

Where can I learn about object-oriented Perl programming?

A good place to start is perlootut, and you can use perlobj for
reference.

A good book on OO on Perl is the ``Object-Oriented Perl''
by Damian Conway from Manning Publications, or ``Intermediate Perl''
by Randal Schwartz, brian d foy, and Tom Phoenix from O'Reilly Media.

Where can I learn about linking C with Perl?

If you want to call C from Perl, start with perlxstut,
moving on to perlxs, xsubpp, and perlguts. If you want to
call Perl from C, then read perlembed, perlcall, and
perlguts. Don't forget that you can learn a lot from looking at
how the authors of existing extension modules wrote their code and
solved their problems.

You might not need all the power of XS. The Inline::C module lets
you put C code directly in your Perl source. It handles all the
magic to make it work. You still have to learn at least some of
the perl API but you won't have to deal with the complexity of the
XS support files.

Download the ExtUtils::Embed kit from CPAN and run `make test'. If
the tests pass, read the pods again and again and again. If they
fail, see perlbug and send a bug report with the output of
"make test TEST_VERBOSE=1" along with "perl -V".

When I tried to run my script, I got this message. What does it mean?

A complete list of Perl's error messages and warnings with explanatory
text can be found in perldiag. You can also use the splain program
(distributed with Perl) to explain the error messages:

perl program 2>diag.out
splain [-v] [-p] diag.out

or change your program to explain the messages for you:

use diagnostics;

or

use diagnostics -verbose;

What's MakeMaker?

(contributed by brian d foy)

The ExtUtils::MakeMaker module, better known simply as ``MakeMaker'',
turns a Perl script, typically called "Makefile.PL", into a Makefile.
The Unix tool "make" uses this file to manage dependencies and actions
to process and install a Perl distribution.

AUTHOR AND COPYRIGHT

This documentation is free; you can redistribute it and/or modify it
under the same terms as Perl itself.

Irrespective of its distribution, all code examples here are in the public
domain. You are permitted and encouraged to use this code and any
derivatives thereof in your own programs for fun or for profit as you
see fit. A simple comment in the code giving credit to the FAQ would
be courteous but is not required.